Tonometric blood pressure measurements is a non-invasive means for continuously monitoring blood pressure (BP) and obtaining additional cardiovascular parameters such as arterial stiffness, cardiac output and stroke volume. Before making such measurement, an accurate position of an artery location is required to be identified over a person's skin.
It is possible to use a single pressure sensor to search for the artery location when the sensor presses on the skin, as in U.S. Pat. No. 8,597,195. During the search, a constant hold-down pressure exerted by the sensor on the skin is required to be maintained. Due to the curvature of a body part under measurement, such as a wrist of a person, the sensor is required to finely and dynamically adjust its position to keep a constant hold-down pressure during the search. A long search time is usually resulted. U.S. Pat. No. 7,771,361 and US20100286538 suggest using an array of optical and pressure sensors to press on the skin to thereby identify the artery location. Although the search time is shorter, accuracy of the artery location is limited by the sensor dimension. High accuracy is achievable only with a small sensor size, the implementation of which is costly.
In obtaining blood pressure measurements, such as through the use of the aforementioned tonometric sensor means, a link between the doctor taking such measurements and higher blood pressure readings has been identified. This link is known as the “white coat effect” or “white coat hypertension” (WCH). The white coat effect, which is believed to be the result of subjects being more nervous when examined by a physician, results in a measured difference above 20 mm Hg in systole and/or 10 mm Hg in diastole over the subject's blood pressure measurement without influence of the effect. Accordingly, it is important to identify whether blood pressure measurement data properly and accurately reflects the situation of the subject and is not affected by phenomena such as the white coat effect.
Techniques for profiling cardiovascular vulnerability to mental stress have been attempted in the art. For example, United States patent application publication number US2008/0081963 describes a method for profiling an individual's vulnerability to detrimental effects of mental stress on vascular function. The disclosed method utilizes a cuff blood pressure sensor for measuring blood pressure at one location on the individual's body and a photoplethysmography (PPG) sensor for measuring blood flow data at a different location on the individual's body, and thus does not provide data which is directly and precisely coupled to the blood pressure data (i.e., the measured data is decoupled in sensed location). Moreover, due to the blood pressure cuff restricting the blood flow during blood pressure measurement, the PPG sensor is unable to measure blood flow data simultaneously with the blood pressure measurement, thereby also decoupling the measured data in time (i.e., decoupled in sensed time). In operation, the method monitors changes in the mental stress levels and changes in the vascular function levels in the individual during a mental stress challenge and correlates the changes of mental stress levels with the changes in vascular function levels to profile the individual's vulnerability to detrimental effects of mental stress on vascular function. The technique does not, however, operate to identify whether blood pressure measurement data properly and accurately reflects the situation of the subject and is not affected by phenomena such as the white coat effect, but instead merely correlates the effect of mental stress with blood pressure.
Other techniques have attempted to monitor variations in the physiological index to initiate blood pressure measurement. For example, United States patent application publication number US2013/0158417 describes a method for non-invasively determining blood pressure by deriving a physiological index from an individual, wherein the physiological index is indicative of a sympathetic activity in the individual, monitoring variations in the physiological index, and instructing a blood pressure determination unit to initiate blood pressure determination when the variations meet a predetermined condition. The method utilizes a PPG sensor at one location on the individual's body and ECG sensors at other locations on the individual's body for measuring heart rate data and a blood pressure sensor located at still another location on the individual's body for measuring blood pressure data, and thus the measured data is decoupled in sensed location. In operation, the method initiates the blood pressure measurement after determining that the monitored variations in the physiological index meet the predetermined condition, thus decoupling the measurements in sensed time. This technique, like the above described technique, does not operate to identify whether blood pressure measurement data properly and accurately reflects the situation of the subject and is not affected by phenomena such as the white coat effect, instead controlling blood pressure measurement in response to certain monitored variations in the physiological index.
Still other techniques have attempted to determine endothelial dependent vasoactivity. For example, U.S. Pat. No. 8,187,196 describes a system having a plurality of sensors disposed at different locations on an individual to extract a pulse-wave time parameter from the signals provided by the sensors. A spectral analyzer of the system then analyzes the pulse-wave time parameter to derive a frequency decomposition which is representative of the endothelial dependent vasoactivity of the individual. As with the methods described above, the described technique does not operate to identify whether blood pressure measurement data properly and accurately reflects the situation of the subject and is not effected by phenomena such as the white coat effect, instead controlling blood pressure measurement in response to certain monitored variations in the physiological index.